1. Field of the Invention
The present invention relates to super hard-highly pure silicon nitrides (Si.sub.3 N.sub.4) and a process and apparatus for producing the same by chemical vapor-deposition. The super hard-highly pure silicon nitrides of the present invention include a preferentially oriented crystalline Si.sub.3 N.sub.4 having a grain size of 1 to 50 .mu.m and a micro Vickers hardness (hereinafter abbreviated as MVH) of more than 3,000 kg/mm.sup.2 under a load of 100 g in which a particular crystal face is oriented in a particular direction, a finely grained crystalline Si.sub.3 N.sub.4 having an average grain size of less than 1 .mu.m and an MVH of more than 3,500 kg/mm.sup.2 under the same load in which a primary cone is composed of these fine grains, and an amorphous Si.sub.3 N.sub.4 having an MVH of more than 2,200 kg/mm.sup.2 under the same load.
2. Description of the Prior Art
Heretofore, it has been known to produce silicon nitrides by a chemical vapor-deposition method. For instance, U.S. Pat. No. 3,226,194 discloses that a mixture of SiN.sub.4 and NH.sub.3 is chemically deposited on a substrate such as graphite, Al.sub.2 O.sub.3, hot-pressed boron nitride and chemical vapor-deposited boron nitride at a temperature of 1,200.degree.-1,900.degree. C. under a gas pressure of less than 300 mmHg to form a crystalline or amorphous silicon nitride having a thickness of less than about 1 mm and that the resulting crystalline Si.sub.3 N.sub.4 has an MVH of 2,850 kg/mm.sup.2 and is used in a high-temperature window, an insulator for fuel cell, a tooling material and an abrasive material.
In U.S. Pat. No. 3,637,423, there is disclosed that a silicon halide (SiX.sub.4) is reacted with ammonia (NH.sub.3) at a low temperature to form an intermediate compound SiX.sub.4.2NH.sub.3 and then the latter compound is evaporated at a temperature of 100.degree.-125.degree. C. under vacuum and chemically deposited on a graphite or a high-melting metal heated at a temperature of 850.degree.-1,385.degree. C., while being carried with N.sub.2 gas; to form a silicon nitride film and that the materials coated with this silicon nitride film have good high-temperature oxidation resistance and insulating property.
In Japanese Patent Application laid open No. 133,199/75, there is disclosed that a mixture of NH.sub.3 and a silicon compound is preheated at 100.degree.-400.degree. C. and then chemically deposited on a substrate heated above 400.degree. C. to form a high density Si.sub.3 N.sub.4 film, which is useful in the semiconductor industry.
In a an article of the title "Pyrolytic Silicon Nitride Coatings", Special Ceramics, Vol. 6, 1973, pp. 305-320, a crystalline or amorphous silicon nitride film having a thickness of 0.8 mm at maximum is produced by reacting SiH.sub.4 or SiCl.sub.4 with NH.sub.3 at a temperature of 800.degree.-1,380.degree. C. and chemically depositing the resulting reaction product on a substrate such as graphite, silicon nitride shaped body or Si with Ar, N.sub.2 or H.sub.2 as a carrier gas under a gas pressure of 1 atm. In this literature, there is disclosed that the substrate coated with this Si.sub.3 N.sub.4 film is excellent in the abrasion resistance, corrosion resistance and high-temperature oxidation resistance, but microcracks are always caused in the amorphous Si.sub.3 N.sub.4 having a thickness of more than 1 .mu.m.
Furthermore, in an article of the title "Some Properties of Vapor Deposited Silicon Nitride Films Using the SiH.sub.4 -NH.sub.3 -H.sub.2 System", Journal of the Electrochemcial Society, Vol. 114, No. 7, 1967, pp 733-737, an amorphous Si.sub.3 N.sub.4 thin film having a thickness of about 9 .mu.m or a whisker-like crystalline Si.sub.3 N.sub.4 is produced by reacting a mixed gas of SiH.sub.4, NH.sub.3 and H.sub.2 as a starting material at a temperature of 600.degree.-1,400.degree. C. and chemically depositing the resulting reaction product on a heated Si substrate. In this literature, there is disclosed that the amorphous Si.sub.3 N.sub.4 thin film has a Young's modulus of 6.7.times.10.sup.3 kg/mm.sup.2 and a Knoop hardness of about 4,000 kg/mm.sup.2 under a load of 8 g. This hardness value corresponds to an MVH of about 2,000 kg/mm.sup.2 under a load of 100 g.
As can be seen from these publications, the micro Vickers hardness of the known chemical vapor-deposited Si.sub.3 N.sub.4 under a load of 100 g is about 2,000 kg/mm.sup.2 at maximum in case of the amorphous body and about 2,850 kg/mm.sup.2 at maximum in case of the crystalline body. However, it is recently desired to develop silicon nitrides having a hardness considerably higher than the above mentioned values. Furthermore, in the well known chemical vapor-deposition of silicon nitrides, the deposition rate is as slow as 0.4 mm/hr at most and hence is impractical. Moreover, silicon nitride having an MVH of about 3,500 kg/mm.sup.2 under a load of 100 g has been produced by the conventional hot pressing method. However, this silicon nitride is obtained only in a block form and is a mixture of .alpha.-Si.sub.3 N.sub.4 and .beta.-Si.sub.3 N.sub.4 and further contains more than 1% of MgO and so on as an impurity. Therefore, this product has a disadvantage that the hardness and mechanical properties at an elevated temperature are considerably deteriorated.